A mobile communications system refers generally to any telecommunications system which enables wireless communication when users are moving within the service area of the system. A typical mobile communications system is a Public Land Mobile Network (PLMN). Often the mobile communications network is an access network providing a user with wireless access to external networks, hosts, or services offered by specific service providers.
Professional mobile radio or private mobile radio (PMR) systems are dedicated radio systems developed primarily for professional and governmental users, such as the police, military forces, oil plants, etc. PMR services have been offered via dedicated PMR networks built with dedicated PMR technologies. This market is divided between several technologies—analog, digital, conventional and trunked—none of which has a dominating role. TETRA (Terrestrial Trunked Radio) is a standard defined by ETSI (European Telecommunications Standards Institute) for digital PMR systems. U.S. Pat. No. 6,141,347 discloses a wireless communications system which uses multicast addressing and decentralized processing in group calls.
One special feature offered by the PMR systems is group communication. The term “group”, as used herein, refers to any logical group of three or more users intended to participate in the same group communication, e.g. call. The groups are created logically, i.e. special group communication information maintained on the network side associates specific user with a particular group communication group. This association can be readily created, modified or canceled. The same user may be a member in more than one group communication group. Typically, the members of the group communication group belong to the same organization, such as the police, the fire brigade, a private company, etc. Also, typically, the same organization has several separate groups, i.e. a set of groups.
A group call typically has a long duration (up to days) during which communication takes place quite infrequently and each interaction is typically short. The total active traffic may be, for example, only 15 minutes during a call. Each talk burst or speech item has an average length of 7 seconds in the existing PMR systems. Therefore, the radio channels or other expensive system resources cannot be allocated all the time, because the service becomes much too expensive. Group communication with a push-to-talk feature is one of the essential features of any PMR network overcoming this problem. Generally, in group voice communication with a “push-to-talk, release-to-listen” feature, a group call is based on the use of a pressel (PTT, push-to-talk switch) in a telephone as a switch: by pressing a PTT the user indicates his desire to speak, and the user equipment sends a service request to the network. The network either rejects the request or allocates the requested resources on the basis of predetermined criteria, such as the availability of resources, priority of the requesting user, etc. At the same time, a connection is established also to all other active users in the specific subscriber group. After the voice connection has been established, the requesting user can talk and the other users listen on the channel. When the user releases the PTT, the user equipment signals a release message to the network, and the resources are released. Thus, the resources are reserved only for the actual speech transaction or speech item.
There are typically various requirements for group communications in communications systems.
Call set up times must be relatively short, i.e. set up times in the order of several seconds cannot be allowed. When a user initiates a call, or rather, a talk item, he/she should be able to start-speaking at the initiation of the set up within few hundreds of milliseconds. The listening parties should hear the talk possibly within approximately a second. This voice delay can be longer because a semi-duplex mechanism is used. These values are only examples.
Group communication requires traffic discipline: one talks and the others listen. Therefore the radio interface is of a semi-duplex type. Only one direction is active at a time. The communications system must be able to control that only one member speaks at a time in a group.
A user can belong to many groups at a same time. Therefore, a communications system must be able to select and prioritize the group the user listens to if there are multiple group communications to the user at the same time.
Not only to traditional PMR users, push-to-talk type of group calls are also attractive to several other types of users, too. For example, private persons might want to have talk groups, such as hobby groups, sport groups, etc. Small business users might also use the push-to-talk type of group communication feature for a more frequent job related communication during a working day within the same work group, either inside the company or within some business community.